Here are some pictures of an idea that has taken shape in my i30 (CRDi) to mount a GPS in the top box.
With security in mind and with ease of setting up, I started by using a washing up pad as a mount with a slot cut at an angle to take the GPS. The GPS measures 120mm x 80mm x 15mm which can be stowed in the top compartment with a squeeze.
Following that, I found a cork based coaster that fitted very neatly into the central part of the compartment - resting on the top of the rubber mat edges and snugly into the hinge area. To this I glued a couple of pieces of plywood to take the GPS snugly. The result is as you see at the bottom picture. That base has to be removed before the compartment can be closed.
The lid of the compartment acts as a shade for better daylight viewing.
BTW there is not enough 'leakage' into the FM antenna to allow this unit to use that facility as it is. When the charge cord is plugged in it's quite OK. As this isn't there normally, I have had to revert to the GPS speaker in lieu (which is still loud enough to warn me when I'm over the speed limit!)